1. Field of the Invention
The present invention relates to an LED substrate and an LED light source device comprising the LED substrate.
2. Description of the Related Art
A liquid crystal display device, wherein liquid crystal per se does not emit light, is provided with a backlight unit functioning as a light source, for example, on a rear section of a liquid crystal panel.
The backlight unit comprises such structural components as a primary light source, a light guide plate, a reflective film, and a lens sheet or a diffuser film, thereby irradiating a display light on a whole surface of the liquid crystal panel.
A conventional example of the backlight unit is a cold cathode fluorescent lamp in which mercury or xenon is sealed in a fluorescent tube as a primary light source. The cold cathode fluorescent lamp, however, had a few problems; inadequate light emission luminance, deterioration of a uniformity ratio due to a low-emission region on the cathode side and short lifetime.
With these conventional technical problems as a driving force, a surface mounting LED having such technical advantages as a high luminance, a long lifetime and a remarkable uniformity ratio is recently widespread as a backlight or a light source of a surface mounting switch.
An LED backlight device comprises a plurality of LED elements aligned in an array or matrix shape. Describing a specific example, a plurality of LED elements mounted on an LED substrate in a direction constitute a light emission unit, and a plurality of the light emission units aligned in a matrix shape constitute an LED backlight device.
The following prior art documents disclose a conventional LED substrate or an LED module (Utility Model Registration No. 3128244, 2008-28171 of the Japanese Patent Application Laid-Open; hereinafter, respectively referred to as Cited Document 1 and Cited Document 2).
FIG. 7 is an illustration of the LED substrate and a method for anchoring the LED substrate recited in the Cited Document 1. A plurality of LED elements 3 are amounted on an LED substrate 101, and tapped holes 102 through which screws are to be inserted are respectively provided in protruding portions 103 of the LED substrate 101 formed in the form of ear on both sides thereof.
When the LED substrate 101 is mounted in a metal cabinet 110, positions of tapped holes 106 in the cabinet 110 and positions of the tapped holes 102 on the substrate 101 are suitably adjusted, and fixing screws 105 are inserted through these holes to be screw-fastened so that the substrate is fixed to the metal cabinet 110. The LED elements 3 are placed so that they are located on through holes 108 in the cabinet 110.
FIG. 8 is a schematic illustration of the fixing screw 105, the tapped hole 106, and a state where the screw 105 is inserted through the tapped hole 106. FIG. 8A is a perspective drawing, and FIG. 8B is a planar drawing in top view.
As illustrated in FIG. 8A, the fixing screw 105 comprises a head portion 111 and a shaft portion 112, and the shaft portion 112 is threaded.
The head portion 111 and the shaft portion 112 both have a cylindrical shape. As illustrated in FIG. 8B, a diameter u3 of the head portion 111 is larger than a diameter u1 of the shaft portion 112. The tapped hole 106 is a through hole having a circular shape in top view. A diameter u2 of the tapped hole 106 is larger than the diameter u1 of the shaft portion 112 and smaller than the diameter u3 of the head portion 111. Practically, u2 has a dimension smaller than and substantially equal to u3.
FIG. 9 is an illustration of a state where the fixing screw 105 is inserted through the tapped hole 102. FIG. 9A shows the state in top view, and FIGS. 9B and 9C show the state in front view.
The LED element 3 is used in, for example, a liquid crystal display device, and the number of the LED elements to be used is on the increase along with the development of a larger liquid crystal panel in recent years. The LED element 3 generates heat when emitting light, and an amount of heat generation inevitably increases with more elements.
In the liquid crystal display device, wherein an enclosed space is formed on the rear side of the liquid crystal panel, heat generated by the LED element stays within the enclosed space, which heats up the liquid crystal device to high temperatures. Further, the heat may cause thermal expansion of the LED substrate 101.
As described earlier, the LED substrate 101 is fixed to the cabinet 110 by the fixing screws 105. In the LED substrate 101 thermally expanded, therefore, any screw-fastened sections are immovable, while otherwise sections are possibly warped as illustrated in FIG. 9C, resulting in the deformation of the substrate 101 in an arch shape.
Accordingly, an intense stress f1 is applied to vicinity of the sections fixed by the screws 105, and cracks may be generated in the substrate 101 or wiring routed in the LED element 3 may be broken in some cases. The wiring breakage disables light emission of the LED element 3 connected to the broken wiring, and the liquid crystal display device thereby lacks uniformity in its luminance.
FIG. 10 is an illustration of the LED substrate recited in the Cited Document 2. An LED substrate 121 comprises five LED elements 3 coaxially provided. These LED elements are connected to one another in series by a wiring 4, and also electrically connected to a power source 7 and an LED driver 8 by way of a substrate connector 5.
The liquid crystal display device, if provided with a plurality of LED elements respectively having different amounts of emission, undergoes luminance non-uniformity. The LED element is characterized in emitting a quantity of light corresponding to a current volume when a voltage applied thereto exceeds a predetermined voltage (forward drop voltage Vf). Therefore, a conventional method for equalizing the quantities of emission of the LED elements is to serially connect the plurality of LED elements as illustrated in FIG. 10 so that a constant amount of current flows in the respective LED elements.
In FIG. 10, an anode of the first LED element 3 is connected to an node Na from a power source 7 by way of the substrate connector 5, and a cathode thereof is connected to an anode of the LED element adjacent thereto. In any LED elements subsequent thereto, cathodes of the previous LED elements are connected to anodes, and anodes of the subsequent LED elements are connected to cathodes. A cathode of the last LED element 3 is connected to a node Nk from the LED driver 8 by way of the substrate connector 5. In FIG. 10, “A” denotes anode, and “K” denotes cathode.
Reference symbols 126a and 126b illustrated in FIG. 10 denote cutout portions (counterbored recesses) into which claws provided in the cabinet are to be fitted for securing the substrate to the cabinet.
To drive one LED substrate 121 having the structure illustrated in FIG. 10, a voltage at least five times as large as the forward drop voltage Vf should be applied to the node Na by the power source 7. An element for low-current drive in the LED driver 8 is connected to the node Nk to draw in a constant current so that the constant current flows in the LED elements 3. As a result, the LED elements 3 emit substantially equal quantities of light, respectively.
As mentioned earlier, increasing dimensions of the liquid crystal panel in recent years demand more LED elements 3. In the case where the LED elements are connected in series to stabilize the luminance, the voltage to be applied by the power source 7 is higher as more elements are directly connected in series. A larger potential difference is generated between the nodes Na and Nk as the applied voltage is increasingly higher, and the substrate connector 5 may result in electrical breakdown. Then, it is no longer possible to supply the constant current to the LED elements, inviting an operation failure. In summary, there should be a limit to the number of the LED elements 3 that can be serially connected.
An alternative method to deal with the disadvantage is, for example, to divide the LED elements 3 serially connected and separately drive the divided LED elements 3 as illustrated in FIG. 11. In FIG. 11, LED substrates on which the LED elements 3 are mounted are divided into an M1 group on left and an M2 group on right facing the drawing. The LED elements 3 on the LED substrate 121 of the M1 group are driven by an LED control circuit 31, and the LED elements 3 on the LED substrate 121 of the M2 group are driven by an LED control circuit 32.
In the illustration of FIG. 11, a plurality of LED substrates 121 illustrated in FIG. 10 are set in the cabinet 130. The LED control circuits 31 and 32 illustrated in FIG. 11 respectively include the power source 7 and the LED driver 8 illustrated in FIG. 10.
In FIG. 10, the substrate connector 5 is provided at one place of the LED substrate 121. In FIG. 11, however, the substrate connected to the LED control circuit 31 (or 32) is provided with the substrate connectors 5 at two places thereof on right and left and thereby connected to the LED control circuit 31 (or 32) by way of one of the connectors and connected to another LED substrate adjacent thereto by way of the other connector.
According to the Cited Document 2, the LED substrate 121 is anchored to the cabinet 130 by anchoring claws 132. As illustrated in FIG. 10, the LED substrates 121 are each provided with the counterbored recesses 126a and 126b into which the anchoring claws 132 are to be fitted. After the anchoring claws 132 are securely fitted into these counterbored recesses, the LED substrate 121 can be immovably mounted on the cabinet 130. In the Cited Document 2, the counterbored recesses are formed at two positions on one side of the LED substrate 121 in a longitudinal direction.
To securely attach the LED substrate 121 to the cabinet 130, the anchoring claws 132 are preferably inserted into the LED substrate 121 through a lower section thereof and further upward in view of gravity. More specifically, in such a component placement in FIG. 11 that an upper side facing the drawing denotes an upward side and a lower side facing the drawing denotes a downward side, the LED substrate 121 and the anchoring claws 132 preferably have a positional relationship as with the M1 group on left facing the drawing.
In FIG. 11, the LED control circuits 31 and 32 are positioned so as to face each other with the LED substrates 121 interposed therebetween. Therefore, there are different positional relationships between the LED control circuits (31 and 32) and the LED substrates 121 respectively included in the M1 group and M2 group.
More specifically, in such a component placement that the upper side facing the drawing denotes the upward side and the lower side facing the drawing denotes the downward side as illustrated in FIG. 11, the anchoring claws 132 are inserted into the LED substrates 121 of the M2 group through upper sections thereof and further downward. As a result, the substrates are less stable than the substrates of the M1 group in view of gravity, possibly causing problems over time; the LED substrates 121 of the M2 group may be largely displaced from their predetermined positions or disengaged from the cabinet 130. These problems unfavorably lead to the luminance non-uniformity of the liquid crystal display device.
An approach for avoiding these problems is to similarly form the counterbored recesses of the LED substrates 121 in the M2 group so as to insert the anchoring claws 132 through the lower sections thereof and further upward. More specifically, two different LED substrates are prepared, wherein different positional relationships are respectively employed in the formation of the substrate connectors 5 and the counteredbored holes.
A more detailed description is given below. As illustrated in FIG. 12A, an LED substrate 121 is produced, wherein the substrate connector 5 is formed on one of short sides of the substrate, and the counterbored recesses 126a and 126b are formed on a long side adjacent clockwise to the short side where the substrate connector 5 is formed. Then, as illustrated in FIG. 12B, an LED substrate 121a is produced, wherein the substrate connector 5 is formed on one of short sides of the substrate, and the counterbored recesses 126c and 126d are formed on a long side adjacent anticlockwise to the short side where the substrate connector 5 is formed.
The substrate 121 illustrated in FIG. 12A is allocated to the M1 group, and the substrate 121a is allocated to the M2 group. According to the structure, the anchoring claws 132 can be inserted into the substrates of both groups through lower sections thereof and further upward as illustrated in FIG. 13. Then, these substrates can be both anchored to the cabinet 130 with more stability.
To accomplish the structure illustrated in FIG. 13, it is necessary to prepare the two different LED substrates as illustrated in FIG. 12. This technical solution, however, invites increase of development costs, for example, it is necessary to prepare a plurality of different metal molds for designing the substrates, and an initial designing cost is additionally necessary.
The structures illustrated in FIGS. 11 and 13 are still unable to avoid the thermal expansion of the substrates due to the heat generation by the LED elements 3 described earlier. The structures illustrated in FIGS. 11 and 13, wherein the substrates are anchored to the cabinet 130 by the anchoring claws 132 alone, includes a high probability of the deformation of the substrates due to thermal expansion, resulting in disengagement of the substrates from the cabinet 130.
A possible way to further increase the stability is to additionally employ the anchoring using screws as illustrated in FIG. 7 in the structure illustrated in FIG. 13. In that case, it is expected that the additional anchoring using the anchoring claws 132 serves to lessen the stress concentrated on vicinity of the screw-fastened sections as compared with FIG. 7. However, a measure of stress concentration is still inevitable notwithstanding the expectation, possibly leading to likelihood of the substrate warp. Moreover, the problem of production cost increase remains unsolved.